The genetic basis of bovine milk production is of immense significance to the dairy industry. An ability to modulate milk volumes and content has the potential to alter farming practices and to produce products which are tailored to meet a range of requirements. In particular, a method of genetically evaluating bovine to select those which express desirable traits, such as increased milk production and improved milk composition, would be desirable.
To date, bovine genomics are poorly understood and little is known regarding the genes which are critical to milk production. While there have been reports of quantitative trait loci (QTLs) on bovine chromosome 20 postulated to be associated with milk production (Georges et al (1995); Arranz et al (1998)), the specific genes involved have not to date been identified due to the poor mapping resolution of current experimental designs (e.g. Mackay 2001; Andersson 2001; Flint and Mott 2001; Mauricio, 2001). Strategies to improve the mapping resolution most often require breeding of large number of progeny to increase the density of cross-overs in the chromosome regions of interest (e.g. Darvasi, 1998). When working with humans or farm animals, this approach is not practical. An alternative approach is linkage disequilibrium (LD) mapping which aims at exploiting historical recombinants and has been shown in some livestock populations, including dairy cattle, to extend over very long chromosome segments when compared to human populations (Famir et al., 2000). However, long range LD is likely to result in a limited mapping resolution and the occurrence of association in the absence of linkage due to gametic association between non syntenic loci. Once mapped, a QTL can be usefully applied in marker assisted selection.
Marker assisted selection, which provides the ability to follow a specific favourable genetic allele, involves the identification of a DNA molecular marker or markers that segregate with a gene or group of genes associated with a QTL. DNA markers have several advantages. They are relatively easy to measure and are unambiguous, and as DNA markers are co-dominant, heterozygous and homozygous animals can be distinctively identified. Once a marker system is established, selection decisions are able to be made very easily as DNA markers can be assayed at any time after a DNA containing sample has been collected from an individual infant or adult animal, or even earlier as it is possible to test embryos in vitro if such embryos are collected.
The applicants have now identified a polymorphism in a gene associated with the QTL effect on bovine chromosome 20.
It is an object of the present invention to provide an application method for marker assisted selection of this polymorphism in the bovine gene which is associated with increased milk volume and altered milk composition; and/or to provide genetic markers for use in such a method; and/or to provide animals selected using the method of the invention as well as milk produced by the selected animals; and/or to provide the public with a useful choice.